Internal combustion engine comprise numberless elements that undergo friction and, as a result, undergo wear because they are subjected to severe stresses when the engine is operating.
One of the ways to guarantee resistance to wear for an element that works sliding so that it can have long/sufficient useful life for the useful-life parameters of the engine is the application of one or more layers of coating on the base metal from which it is built. The coating, developed specifically for resisting wear and abrasion, maintains the performance properties of the element that works sliding, even after millions of explosion cycles of the engine.
Besides cylinders, pistons and panels, an internal combustion engine has a number of additional elements provided with at least one slide surface, which can receive coatings so as to prolong the useful life of the piece. Some of these elements are bearings, components of the valve gear (tappets, camshafts, lobes, etc.).
In this regard, there are numberless techniques using the most varied compositions of coatings and numberless application processes, each trying to optimize the performance and durability properties of the most varied types and configurations of elements for use on an internal combustion engine having a slide surface.
The most relevant prior-art coatings to the present invention are the ceramic coatings composed of nitrides generated through a physical-deposition process in the vapor phase (physical vapor deposition), called herein PVD.
Nitride coatings formed through PVD processes exhibit inner tensions that increase as a function of the thickness of the layer (see FIG. 1). These tensions are typically compressive and become more compressive as the film grows.
FIG. 1 presents a graph that evidences the increase in compressive inner tension in a chrome-nitride (CrN) coating deposited at 450° C. and with a tension of acceleration of particles against the substrate of 175V, this coating having been obtained by the sputtering deposition of a material onto the target surface.
This phenomenon of increase in the compressive inner tension as a function of the growth of the coating is due to the fact that in the plasma environment the positive ions are attracted by an electric and magnetic field to the surface on which the coating grows, so that the energy of bombing ions onto the coating induces the generation of compressive tensions in the films.
Moreover, nitride films are crown in an environment that generally involves a few hundreds of degrees centigrade, wherein, after deposition of the coating onto the substrate, the film is cooled and, as a result, the existence of different dilatation coefficients promotes greater contraction of the substrate. Thus, different dilatation between the coating material and substrate material promotes the “formation” of compressive inner tensions in the nitride films.
The existence of such compressive inner tensions brings various drawbacks, limiting, on the one hand, the coating thickness and decreasing, on the other hand, the work load that such elements provided with coating may bear. It should be noted that, since the inner tensions are compressive, they add to the loading values to which the elements are subject.
As a result, every coating, upon being stressed by an external loading (due to the application as a component of a machine or an internal combustion engine, for example), will be subjected to the generation of tensions due to the loading and then the closer to the neutral value (or null inner tension) the initial tension of the coating is, the greater the loading which the latter bears will be, until it reaches a tension that causes rupture of the coating (crack nucleation and growth thereof). When the values of the compressive inner tension and of loading exceed the limit values which the coating can receive, there is a displacement of the coating, which may cause severe damages to the coating.
With regard to coating growth, one observes that some coatings, for example, TiN, exhibit limitation in the growth due to the high tension thereof and, as a result, it is not possible to grow the coating to thicknesses higher than 2 or 3 micrometers (μm).
Another condition that also affects the resistance of the films is the presence of pores. Pores are regions present in the coating, which have defect due to the extremely low localized cohesion of the material. Thus, the pores act as tension concentrators in greater or lesser scale, depending on the geometry and number of pores. Naturally, a high porosity rate impairs the resistance of a coating to wear.
In this context, the present-day model in nitrided ceramic coatings is reflected by the following ratio between the variables that interact in the coating. A larger thickness of layer will generate higher inner compression tension, which aggravates with the porosity rate of the coating. This ratio ends up limiting the values of hardness and the consequent resistance to wear of the solutions indicated by the prior art.
An example of this are documents U.S. Pat. No. 5,449,547 and U.S. Pat. No. 6,270,081, which describe piston rings provided with CrCN/CrON and CrN coatings, respectively. It should be noted that the hardness of these coatings is limited to 2200 HV and 1800 HV, respectively, the second document mentioning porosity rates higher than 3%.
In turn, document U.S. Pat. No. 6,372,369 discloses coatings for piston rings made of CrN and TiN with hardness that ranges from 1300 to 2300 HV. It should be noted that the hardness value is hardly higher than 200 HV when dealing with nitrided mono-layer coatings, this hardness ceiling being one of the limitations resulting from the model relationship existing for this type of coatings.
Unfortunately, present-day coatings of elements that work sliding do not exhibit an adequate performance of resistance to wear when stressed at severe working conditions. Such reality indicates that present-day PVD coatings will not work correctly in the next few generations of engines, especially on engines having exhaust gas recirculation—EGR—and selective catalytic reduction—SCR, and these engines will be used in future generations with a view to reduce the pollutant emission rates.
Thus, the conventional methods presented in the prior art do not disclose a technical solution that will enable one to obtain a nitrided ceramic coating with high properties, which simultaneously presents the possibility of causing a film to grow, while keeping the compressive inner tensions reduced, capable of enabling an extraordinary resistance to wear.